
public class Test19091805 {
	public static void main(String[] args) {
		System.out.println(factorial(20));
	}

	private static long factorial(int a) {
	long s=0;
	if(a==0) s=0;
	if(a==1) s=1; 
	else if(a>=2){
	s=a * (factorial(a-1)-factorial(a-2)) + factorial(a-1);
	}
	return s;
	}
}
	

	
	       
